Pseudotachylytes and Earthquake Source Mechanics

Destructive earthquakes nucleate at depth (10 to 15 km), therefore monitoring active
faults at the Earth’s surface, or interpreting seismic waves, yields only limited information
on earthquake mechanics. Tectonic pseudotachylytes (solidified friction-induced
melts) decorate some exhumed ancient faults and remain, up to now, the only fault
rocks recognized as the unambiguous signature of seismic slip. It follows that pseudotachylyte-
bearing fault networks might retain a wealth of information on seismic faulting
and earthquake mechanics. In this chapter, we will show that in the case of large
exposures of pseudotachylyte-bearing faults, as the glacier-polished outcrops in the
Adamello massif (Southern Alps, Italy), we might constrain several earthquake source
parameters by linking field studies with microstructural observations, high-velocity
rock friction experiments, modeling of the shear heating and melt flow, and dynamic
rupture models. In particular, it is possible to estimate the rupture directivity and the
fault dynamic shear resistance. We conclude that the structural analysis of exhumed
pseudotachylyte-bearing faults is a powerful tool for the reconstruction of the earthquake
source mechanics, complementary to seismological investigations.